Flocculants are reagents which when added to suspensions of solids cause the solids to aggregate into larger particles which settle more rapidly and efficiently than in an untreated suspension. Polymeric water soluble flocculants may have a nonionic, anionic, cationic, or amphoteric character. The particular type of flocculant to choose in a given application depends primarily on the physical and chemical nature of the suspended solids, as well as the pH of the aqueous slurry.
Kaolinic clays are mined in the southeastern United States and are usually mined by the open pit method. The clay is initially slurried in water at about 30-50% solids, dispersed with various combinations of inorganic and organic dispersants to maintain a low viscosity and processed. Some of the processing may include the removal of magnetic impurities, removal of sand and silt, improvement of the whiteness or brightness, and dewatering (or filtration). Dewatering is commonly accomplished by rotary vacuum filters, although other methods of dewatering such as pre-thickening, centrifugation, and filtration on pressure filters are feasible. The filter cake obtained from a rotary vacuum filter is, in the present state of the art, usually about 50% to 60% solids.
For the dewatering step of clay processing, it has been found useful to add a flocculating agent as a filter aid. Many high molecular weight organic polymers, whether cationic, anionic or nonionic, have been shown to be capable of flocculating kaolin clay in a satisfactory manner, but they have generally not been able to do so without having deleterious effects on the viscosity of final product, high solids dispersions (e.g., near 70% solids) subsequently prepared from the filter cake. In the present state of the art, a portion of the filter cake is typically spray dried and blended back with the remaining filter cake to obtain an aqueous slurry containing about 70% solids or more. Additional dispersants may be added to the 70% solids slurry with mixing as needed to obtain a final Brookfield viscosity of less than 1000 milliPascal-seconds (mPa.s), and preferably less than 500 mPa.s. This low viscosity 70% solids slurry may then be shipped in bulk to a customer. The presence of a flocculant in the slurry often works against the desired effect of the dispersant, typically a low molecular weight polyacrylic acid, to disperse the solids and lower the viscosity. The necessary low viscosity for the final product is then not achieved, and a solids content higher than 60% is difficult to obtain.
An object of the filtering process is to produce a compact filter cake containing as little water as possible, while permitting the water to be removed from the cake as quickly as possible. In addition, it is desirable to minimize the amount of solids carried off with the water, since those solids are lost product. Therefore, a filter aid desirably binds the clay solids into flocs which are easily dewatered with minimal loss of solids and maximum solids content in the filter cake. As discussed above, after filtering, at least a portion of the filter cake may be redispersed into slurry form, usually by the use of a dispersant, and spray dried. The spray dried product, which may be combined with a portion of the filter cake, is then redispersed to produce a low viscosity 70% slurry which can be shipped in this form to customers. Because filter aids are flocculants, they may interfere with the dispersal of solids after filtering. It is therefore important that the filter aid not interfere with the redispersal of the clay slurry downstream of the filtering process.
Sharpe, U.S. Pat. Nos. 4,647,382, 4,741,838, 4,990,262 and 4,990,263 all relate to the use of various organic polymeric flocculants as filter aids for kaolinitic clays. U.S. Pat. No. 4,647,382 relates to the use of a polymer comprising 50-99 mole percent repeating units derived from acrylamide and 1-50 mole percent of repeating units derived form 2-acrylamido-2-methyl propyl sulfonic acid, or salts thereof. U.S. Pat. No. 4,741,838 relates to the use of an anionic polymeric flocculant comprising 1-100 mole percent of repeating units derived from 2-acrylamido-2-methyl propyl sulfonic acid, or a salt thereof, and may also include 75-99 mole percent repeating units of acrylamide, and up to 10 mole percent of a copolymerizable monoethylenically unsaturated monomer, such as acrylic acid. The flocculants used in both of these patents are anionic polymers. U.S. Pat. Nos. 4,990,262 and 4,990,263 relate to the use of certain cationic polymers as flocculants for dewatering filter cakes. The U.S. Pat. No. 4,990,262 uses a condensation product of formaldehyde and dicyandiamide, while the U.S. Pat. No. 4,990,263 uses copolymer of melamine and formaldehyde.
So-called "selective flocculation" is a procedure that is widely used commercially to separate finely divided minerals and powders. In the case of clay, some procedures utilize anionic polymers to selectively flocculate the clay, leaving the impurities dispersed and amenable to subsequent separation. Commercial variants of selective flocculation employ weakly anionic polymers such as hydrolyzed polyacrylamide to selectively flocculate impurities in the clay, leaving the purified clay dispersed. See, for example, U.S. Pat. No. 3,837,482, Sheridan, U.S. Pat. Nos. 3,701,417 and 3,862,027, both Mercade, U.S. Pat. No. 3,371,988, Maynard, et al., and U.S. Pat. No. 4,604,369, Shi.
Polyacrylic acids are anionic polymers derived from acrylic acid monomer. At relatively low molecular weights of about 2,000 to 10,000, they are commonly used as deflocculating or dispersing agents for kaolin clays. In particular, low molecular weight polyacrylic acid is commonly used to redisperse a flocculated filter cake. High molecular weight polyacrylic acids (with molecular weights over 2,000,000), however, have just the opposite effect. They have been found to act as flocculants.
In commonly assigned Behl, et al., U.S. Pat. No. 5,535,890, incorporated herein by reference, a high molecular weight polyacrylic acid or a salt thereof, or a or polyacrylic acid copolymer, is used as a flocculating agent for, among other purposes, flocculating impurities, such as metal oxides, for removal from a kaolin slurry. In that process, the flocculation is conducted under very alkaline conditions, the pH being about 9-11, at which the polymer is found to selectively flocculate metal oxide impurities without flocculating the slurried kaolin clay.